Chapter 1 :: Introduction Programming Language Pragmatics Michael L. Scott Programming Languages What programming languages can you name? Which do you know? 1
Introduction Why are there so many programming languages? evolution -- we've learned better ways of doing things over time socio-economic factors: proprietary interests, commercial advantage orientation toward special purposes orientation toward special hardware diverse ideas about what is pleasant to use Introduction What makes a language successful? easy to learn (BASIC, Pascal, LOGO, Scheme, Alice) easy to express things, easy use once fluent, "powerful (C, Common Lisp, APL, Algol-68, Perl) easy to implement (BASIC, Forth) possible to compile to very good (fast/small) code (Fortran) backing of a powerful sponsor (COBOL, PL/1, Ada, Visual Basic, C#) wide dissemination at minimal cost (Pascal, Turing, Java, Alice) 2
Blockly Screenshot Introduction Why do we have programming languages? What is a language for? way of thinking -- way of expressing algorithms languages from the programmer s point of view abstraction of virtual machine -- way of specifying what you want the hardware to do without getting down into the bits languages from the implementor s point of view 3
Why study programming languages? Help you choose a language. C vs. Modula-3 vs. C++ for systems programming Fortran vs. APL vs. Ada for numerical computations Ada vs. Modula-2 for embedded systems Common Lisp vs. Scheme vs. ML for symbolic data manipulation Java vs. C/CORBA for networked PC programs Why study programming languages? Make it easier to learn new languages some languages are similar; easy to walk down family tree concepts have even more similarity; if you think in terms of iteration, recursion, abstraction (for example), you will find it easier to assimilate the syntax and semantic details of a new language than if you try to pick it up in a vacuum. Think of an analogy to human languages: good grasp of grammar makes it easier to pick up new languages (at least Indo-European). 4
Why study programming languages? Help you make better use of whatever language you use understand obscure features: In C, help you understand unions, arrays & pointers, separate compilation, varargs, catch and throw In Common Lisp, help you understand first-class functions/closures, streams, catch and throw, symbol internals Why study programming languages? Help you make better use of whatever language you use (2) understand implementation costs: choose between alternative ways of doing things, based on knowledge of what will be done underneath: use simple arithmetic e.g.(use x*x instead of x**2) use C pointers or Pascal "with" statement to factor address calculations avoid call by value with large data items in Pascal avoid the use of call by name in Algol 60 choose between computation and table lookup (e.g. for cardinality operator in C or C++) 5
Why study programming languages? Help you make better use of whatever language you use (3) figure out how to do things in languages that don't support them explicitly: lack of suitable control structures in Fortran use comments and programmer discipline for control structures lack of recursion in Fortran, CSP, etc write a recursive algorithm then use mechanical recursion elimination (even for things that aren't quite tail recursive) Why study programming languages? Help you make better use of whatever language you use (4) figure out how to do things in languages that don't support them explicitly: lack of named constants and enumerations in Fortran use variables that are initialized once, then never changed lack of modules in C and Pascal use comments and programmer discipline lack of iterators in just about everything fake them with (member?) functions 6
Language Categories Two common language groups Imperative von Neumann (Fortran, Pascal, Basic, C) object-oriented (Smalltalk, Eiffel, C++, Java) scripting languages (Perl, Python, JavaScript, PHP) Declarative functional (Scheme, ML, pure Lisp, FP) logic, constraint-based (Prolog, VisiCalc, RPG) Imperative languages Imperative languages, particularly the von Neumann languages, predominate They will occupy the bulk of our attention We also plan to spend time on functional, logic languages 7
Compilation vs. interpretation not opposites not a clear-cut distinction Pure Compilation The compiler translates the high-level source program into an equivalent target program (typically in machine language), and then goes away: Pure Interpretation Interpreter stays around for the execution of the program Interpreter is the locus of control during execution 8
Interpretation: Greater flexibility Better diagnostics (error messages) Compilation Better performance Common case is compilation or simple preprocessing, followed by interpretation Most language implementations include a mixture of both compilation and interpretation 9
Note that compilation does NOT have to produce machine language for some sort of hardware Compilation is translation from one language into another, with full analysis of the meaning of the input Compilation entails semantic understanding of what is being processed; pre-processing does not A pre-processor will often let errors through. A compiler hides further steps; a pre-processor does not Many compiled languages have interpreted pieces, e.g., formats in Fortran or C Most use virtual instructions set operations in Pascal string manipulation in Basic Some compilers produce nothing but virtual instructions, e.g., Pascal P-code, Java byte code, Microsoft COM+ 10
Implementation strategies: Preprocessor Removes comments and white space Groups characters into tokens (keywords, identifiers, numbers, symbols) Expands abbreviations in the style of a macro assembler Identifies higher-level syntactic structures (loops, subroutines) Implementation strategies: Library of Routines and Linking Compiler uses a linker program to merge the appropriate library of subroutines (e.g., math functions such as sin, cos, log, etc.) into the final program: 11
Implementation strategies: Post-compilation Assembly Facilitates debugging (assembly language easier for people to read) Isolates the compiler from changes in the format of machine language files (only assembler must be changed, is shared by many compilers) Implementation strategies: The C Preprocessor (conditional compilation) Preprocessor deletes portions of code, which allows several versions of a program to be built from the same source 12
Implementation strategies: Source-to-Source Translation (C++) C++ implementations based on the early AT&T compiler generated an intermediate program in C, instead of an assembly language: Implementation strategies: Bootstrapping Early Pascal compilers built around a set of tools that included: A Pascal compiler, written in Pascal, that would generate output in P-code, a simple stack-based language A Pascal compiler already translated into P-code A P-code interpreter, written in Pascal Compiler.p We have to write this Compiler.pcode Interpreter.p P-code interpreter translated to C Interpreter.exe 13
Pascal Interpeter Compiler.pcode Interpreter.exe Program.p Program.pcode Interpreter.exe Output of Program.p Bootstrap compiler Modify Compiler.p to compile to native code instead of P-code, then use the compiler to compile itself Compiler.p Compiler.p to x86 run via Interpreter X86 Compiler.exe Program.p Program.exe 14
Implementation strategies: Compilation of Interpreted Languages The compiler generates code that makes assumptions about decisions that won t be finalized until runtime. If these assumptions are valid, the code runs very fast. If not, a dynamic check will revert to the interpreter. Implementation strategies: Dynamic and Just-in-Time Compilation In some cases a programming system may deliberately delay compilation until the last possible moment. Lisp or Prolog invoke the compiler on the fly, to translate newly created source into machine language, or to optimize the code for a particular input set. The Java language definition defines a machine-independent intermediate form known as byte code. Byte code is the standard format for distribution of Java programs. The main C# compiler produces.net Common Language Runtime (CLR), which is then translated into machine code immediately prior to execution. 15
Compilers exist for some interpreted languages, but they aren't pure: selective compilation of compilable pieces and extrasophisticated pre-processing of remaining source. Interpretation of parts of code, at least, is still necessary for reasons above. Unconventional compilers text formatters silicon compilers query language processors Programming Environment Tools Tools; Integrated in an Integrated Development Environment (IDE) 16
Phases of Compilation Scanning: divides the program into "tokens", which are the smallest meaningful units; this saves time, since character-by-character processing is slow we can tune the scanner better if its job is simple; it also saves complexity (lots of it) for later stages you can design a parser to take characters instead of tokens as input, but it isn't pretty scanning is recognition of a regular language, e.g., via DFA (deterministic finite automaton) 17
Parsing is recognition of a context-free language, e.g., via Pushdown Automaton (PDA) Parsing discovers the "context free" structure of the program Informally, it finds the structure you can describe with syntax diagrams (the "circles and arrows" in a Pascal manual) Pascal Railroad diagram 18
Semantic analysis is the discovery of meaning in the program The compiler actually does what is called STATIC semantic analysis. That's the meaning that can be figured out at compile time Some things (e.g., array subscript out of bounds) can't be figured out until run time. Things like that are part of the program's DYNAMIC semantics Intermediate form (IF) done after semantic analysis (if the program passes all checks) IFs are often chosen for machine independence, ease of optimization, or compactness (these are somewhat contradictory) They often resemble machine code for some imaginary idealized machine; e.g. a stack machine, or a machine with arbitrarily many registers Many compilers actually move the code through more than one IF 19
Optimization takes an intermediate-code program and produces another one that does the same thing faster, or in less space The term is a misnomer; we just improve code The optimization phase is optional Code generation phase produces assembly language or (sometime) relocatable machine language Certain machine-specific optimizations (use of special instructions or addressing modes, etc.) may be performed during or after target code generation Symbol table: all phases rely on a symbol table that keeps track of all the identifiers in the program and what the compiler knows about them This symbol table may be retained (in some form) for use by a debugger, even after compilation has completed 20
Lexical and Syntax Analysis GCD Program (Pascal) Lexical and Syntax Analysis GCD Program Tokens Scanning (lexical analysis) and parsing recognize the structure of the program, groups characters into tokens, the smallest meaningful units of the program 21
Lexical and Syntax Analysis Context-Free Grammar and Parsing Parsing organizes tokens into a parse tree that represents higher-level constructs in terms of their constituents Potentially recursive rules known as context-free grammar define the ways in which these constituents combine Context-Free Grammar and Parsing Example (Pascal program) 22
Context-Free Grammar and Parsing GCD Program Concrete Parse Tree Next slide Context-Free Grammar and Parsing GCD Program Parse Tree (continued) 23
Syntax Tree GCD Program Abstract Parse Tree Code Generation Naïve MIPS assembly code fragment addiu sp, sp, -32 # Reserve room for local vars sw ra, 20(sp) # save return address jal getint # read nop sw v0, 28(sp) # store i jal getint # read nop sw v0, 24(sp) # store j lw t6, 28(sp) # load i to t6 lw t7, 24(sp) # load j to t7 nop beq t6, t7, D # branch if I = J nop A: lw t8, 28(sp) # load I... 24